As is known, ingot molds in common use in the steel industry have a tapered inner cavity; and if the end of the cavity of largest cross-sectional area is at the bottom of the mold, it is classified as a big-end-down type of mold. As molten steel is poured into the ingot mold, the metal nearest the mold wall is cooled first to form the skin of the mold. Thereafter, as more heat is extracted from the metal, this skin grows in thickness until the entire ingot has solidified. Due to the shrinkage of the metal during solidification, a pipe or shrinkage cavity is formed at the top of the ingot; and this shrinkage cavity most be cropped off after the ingot is formed into a slab in a blooming mill and before rolling into hotband. Obviously, it is desirable to minimize the depth of the primary pipe or shrinkage cavity in order to increase the yield obtained from the ingot.
In order to minimize the length of the primary pipe and thus minimize slab crop losses, molds have been used utilizing "bottle top" configuration wherein the top end of a big-end-down mold cavity is necked down in the area where the primary pipe forms. This relocates a portion of the shrinkage cavity or pipe, decreasing the primary pipe region and creating or increasing a secondary pipe cavity which welds together during hot-working from ingot to hot-band. It is important, in cropping off the primary pipe cavity, to insure that the crop shear does not sever the secondary pipe cavity. If it is severed, air will flow into the secondary cavity and oxidize the metal with the result that scale will be included in the interior of a slab when it is rolled into strip.